The present invention relates to a semiconductor nonvolatile memory for use in electronic instruments such as a computer.
The semiconductor nonvolatile memory of, particularly, the MOS type operates to carry out memory storing by utilizing channeling hot electrons or tunneling current so as to change a potential level of a floating gate electrode. FIG. 3 shows a rewriting performance of the general semiconductor nonvolatile memory. The floating gate electrode of the memory device changes its potential level according to different bits of data "1" and "0" between +V.sub.TFG. This potential level difference is called normally the writing window of the nonvolatile memory. The width of the window tends to gradually reduce with repetition of. rewriting operation due to carrier trapping which is induced while electrons pass through a thin gate insulating film, thereby limiting the allowable maximum cycle number of rewriting operations. Though, normally the nonvolatile memory of the MOS type has the maximum rewriting cycle number in the order of 10.sup.5 -10.sup. 6, there is need for a further increase in the maximum rewriting cycle number in the memory market.
FIG. 2 is a structural block diagram showing the conventional semiconductor nonvolatile memory. A nonvolatile memory array 111 is connected to a row decoder 102, a column decoder 103 and an I/O circuit 104 containing a sensing amplifyer. Further, the I/O circuit 104 is connected to a programing pulse generating circuit 122. Further, there are provided address input terminals A.sub.O -A.sub.n, data input/output terminals I/O.sub.1 -I/O.sub.m, a data reading terminal R and a programing control terminal STO.
Conventionally, the use of a nonvolatile memory array is terminated or stopped when the statistically determined maximum rewriting cycle number is reached without regard to the actual internal state of the array. Otherwise, the degradation degree of a memory array may be monitored by means of a monitor element which has the same rewriting performance as that of the memory array.
Conventionally, when significantly increasing the maximum rewriting cycle number, construction and fabrication process of the nonvolatile memory array must be upgraded.